Firearm (and air gun) sighting systems that simply align a forwardly positioned element with a rearwardly positioned element are generally known as “iron” sights and exclude the use of optics, such as telescopic or reflex sights. Typically, the forward sight element is a post, bead, or blade that is visually aligned with a notch or circular hole that is part of the rear sight element. A rear sight that uses a circular opening is often referred to as an aperture sight. In many cases, the forward element is vertically adjustable so that alignment of the sights can be vertically adjusted relative to the bore of the firearm barrel (elevation adjustment). Also in many cases, the rear sight element is adjustable side to side so that alignment of the two sight elements can be adjusted horizontally relative to the barrel bore axis (windage adjustment).
Aperture sights utilize a tendency of the human eye to automatically center the front sight when looking through the rear aperture. A larger aperture allows the shooter to more quickly align the sights, while a smaller aperture allows the shooter to more precisely align the sights. Some aperture sights allow the user to select the aperture size, depending on the shooting situation or environmental conditions, thus giving the aperture sighting system more flexibility and a wider range of utility.
Iron sights are inherently more durable than optical sights, which include glass lenses that can be damaged, or reflex sights, which require electrical power for illumination. Some users of optical or reflex sights also equip their firearm with a set of back-up iron sights (BUIS), which may be used under conditions where the optical or reflex sight is either inappropriate or inoperable. In some cases, the back-up iron sights are positioned in alignment with an optical sight, but are foldable in order to maintain a lower profile until deployment is needed.
Prior foldable back-up iron sights in which the rear element includes an adjustable size aperture sight, the adjustment is made by rotating elements having varied aperture size into and out of the line of view. These rotating mechanisms typically occupy a larger amount of space than a single aperture sight or require the element to be moved to a particular aperture setting when the sight is folded to a reduced profile position.